Will New York's super-size drink ban work?

Yet other observational studies have found no evidence that sugar-sweetened drinks are especially fattening.

To resolve these discrepancies, scientists produced a series of meta-analyses that looked for patterns across the whole research literature.

In 2006, Vasanti Malik and Frank Hu of the Harvard School of Public Health summed the results from 30 different studies, and found that half of them showed an effect significant enough for them to conclude that "the weight of epidemiologic and experimental evidence" supports a link between sugar-sweetened beverages and obesity.

As was the case with the observation studies, though, meta-analyses from other groups came up with different results. A review from last year, by Richard Mattes and David Allison, looked at the same corpus as Malik and Hu but found the evidence inconclusive. Another review from 2007 concluded from 88 studies that there were "clear associations" between soft drinks and body weight. A review from 2008 combined a dozen studies of children and adolescents, and found an association that was "near zero."

The inevitable next step came in 2011, with the publication of a paper that tried to review the quality of all these reviews – a meta-meta-analysis. The authors of this study, led by Douglas Weed, gathered 17 omnibus accounts of sugar-sweetened beverage research and tested each one according to a standard template. On average, they said, the papers used just "4 of 11 best practices for an objective and systematic methodology".

The implication was clear: research on sugar-sweetened beverages has been so sloppy and vague as to be inconclusive. But Malik and Hu responded with an editorial pointing out the biases in Weed's own methodology: "Rather than shedding more light on this pressing public health issue," they wrote, "[it] obscured important relations."

One hopes the Harvard team is about to publish their own meta-meta-analysis, so that another group can compare the two for the world's first meta-meta-meta-analysis.

Unanswered questions

But before you throw your hands up in despair, there's one more set of facts to consider. Weed and his co-authors work as private research consultants, and their 2011 paper was funded by the Coca-Cola Company (Weed has given seminars for Coca-Cola's Beverage Institute that are designed to teach doctors "what recommendations should and should not be made to consumers as a result of epidemiological findings"). The 2008 review that found a "near zero" effect of sugar-sweetened beverages was funded by a grant from the American Beverage Association, which ended up hiring its senior author to a full-time position (there were some methodological problems with that paper too).

Neither Richard Mattes nor David Allison received industry funding for their sceptical meta-analysis, but both have taken grants, honoraria and consulting fees from numerous food and beverage companies.

The scientists who have lined up against the sugary-drink hypothesis are largely the ones with financial links to the beverage industry. And while that's not proof of corruption – lots of researchers take private money, and there's evidence of both black-hat and white-hat bias in the obesity literature – their approach fits a disturbing pattern: by counselling caution and harping on about uncertainty, the industry consultants are appealing to a paranoid style of American science that (intentionally or not) serves to stave off government regulation.

But even if you're inclined to believe the toxic-soda theory, important questions remain. Can targeted interventions to reduce soft-drink consumption really prevent weight gain? We've seen only a few randomised, controlled trials, and their results are ambiguous.

Would drinking less soda make everyone healthier, or just some people? The data suggest that little kids are better than adolescents at regulating their liquid-calorie intake, and some adults may be more vulnerable to soft drinks than others.

Finally, what would happen if we did nothing at all? Carbonated soft-drink sales have been in decline since the late 1990s even without a soda tax or size restriction. A study from last year found that Americans' intake of added sugar has dropped by almost 25 per cent in recent years. Meanwhile, obesity rates across the whole population seem to be flattening out.

Bloomberg's public-health officials argue that it is better to act now; the beverage industry would rather they waited for more research (and counter-research) to fill out the details. That's the science – the rest is politics.

This article first appeared in Slate magazine. Daniel Engber is a columnist for Slate, and has previously written on obesity and healthcare reform

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